The present invention relates to an optical position sensing device which allows a displacement along a two-dimensional plane to be determined.
The encoder enables a signal interpretation such as to obtain information on a position, velocity, an acceleration and/or the like when the encoder works in pair with a codewheel or a codestrip. The codewheel/codestrip comprises a regular pattern of slots and bars. According to the position of the slots and bars, the codewheel/codestrip permits or prevents light from passing through. The optical detector detects the light that is transmitted by the codewheel/codestrip and provides an unambiguous information on the motion of the codewheel/codestrip based on the detected light signal.
FIG. 1A shows a cross-section of a typical optical encoder 100. The encoder 100 comprises of a housing 104, an optical emitter 101, an optical detector 102 and an optical lens 106.
A free area 107 is provided in the housing between the optical emitter 101 and the optical detector 102. A part of a codewheel 103 is accommodated within the free area 107, such that it is able to interfere with the path of the light 105 emitted by the optical emitter 101. The codewheel 103 is able to move freely within the free area 107 and the light 105 from the optical emitter 101 is able to pass through or is prevented from passing through the pattern on codewheel 103. The light 105 that passes through the codewheel 103 is detected by the optical detector 102, which produces a corresponding photocurrent.
FIG. 1B shows a cross-section of a typical reflection, optical encoder 120. The reflection encoder 120 comprises the optical emitter 101, the optical detector 102, a first lens 110 and a second lens 111.
The first lens 110 is provided directly next to the optical emitter 101 for collimating the light emitted by the optical emitter 101 into parallel light beam 112. The parallel light beam 112 is directed towards the codewheel/codestrip 103, and depending on the patterns on the codewheel/codestrip 103, part of the parallel light beam 112 is either absorbed or reflected. The reflected light beam 113 is directed towards the second lens 111 which is provided directly next to the optical detector 102, wherein the second lens 111 focuses the reflected light beam 113 onto the optical detector 102. The optical detector 102 detects the amount of light received and generates a corresponding photocurrent.
The output of the optical detector in both the above encoders, that is the photocurrent, is normally processed in an analog signal processor to generate an analog signal, and the analog signal is subsequently passed to an Analog-to-Digital Converter (ADC) for generating digital outputs, providing information on the magnitude and direction of the displacement of the codewheel, and hence a device the codewheel is coupled to.
ADC circuits are usually very large and a number of discrete output levels are needed to represent the displacement. A discrete range of reference thresholds, depending on the number of output levels required, are therefore needed to be set to discriminate between the output levels. The reference thresholds are to be designed such that they are tolerant to any changes in the photocurrent levels due to changes in the brightness of the light source, in particular the optical emitter. The reference thresholds must also be tolerant to any variations in the fabrication process of the photodetectors, aging of any devices used and any other transient factors like temperature shift.
To overcome the problems mentioned above, an optical rotary pulse generating encoder with quadrature output is commonly used as a digital optical encoder. In an optical rotary pulse generating encoder with quadrature outputs, the optical detector usually comprises multiple sets of photodiodes as photodetectors, and the photocurrents generated by the photodiodes are fed through signal processing circuitries to produce a plurality of pairs of complementary analog signals. These pairs of complementary analog signals are further processed, for example in comparator circuits, to produce digital output signal pairs which are in quadrature. The magnitude and direction of displacement can be extracted from the quadrature output signal pairs, providing the displacement information from an initial position.
The HEDR-8000 series optical encoders manufactured by Hewlett Packard have an arrangement similar to the encoder 120 described in FIG. 1B. The optical detectors used in the optical encoders comprise of four photodetectors which are illuminated by light reflected from a codewheel in an alternating manner. The photocurrents generated by the photodetectors are compared, and a pair-of output signals which is indicative of the position of a shaft is generated.
The output signals produced by either the quadrature output encoder described above or the optical encoders manufactured by Hewlett Packard are only able to provide information on displacement along a single axis, i.e. for single axis applications. For dual-axis applications like detecting the movement of a trackball of a mouse, two separate optical rotary pulse generating encoders are needed in order to provide displacement information of the mouse along two axes, or on a two-dimensional plane. This results in more piece parts and larger operational space of the encoder, hence the product cost is increased. Therefore, an efficient digital optical position sensing device for sensing a two-dimensional displacement is desired.